1). Field of the Invention
The present invention generally relates to a DC-to-DC converter. More specifically, the present invention is directed to a voltage drop type DC-to-DC converter capable of preventing overvoltages appearing in an input voltage and an output voltage.
2). Description of the Prior Art
Portable type electronic appliances such as notebook type personal computers install cells as power supplies thereof. To operate these portable type electronic appliances under stable conditions, these cells are preferably capable of supplying constant voltages.
To the contrary, general-purpose cells inherently owns such a characteristic that output voltages are gradually lowered during discharge operation. Therefore, portable type electronic appliances are equipped with DC-to-DC converters capable of maintaining the output voltages of these cells at constant values.
The operation time during which an electronic appliance can be effectively operated by a cell is one of the major important factors to evaluate the performance of this portable type electronic appliance. To maintain such an effective operation time as long as possible, power consumption by this portable type electronic appliance should be, of course, reduced, and furthermore, the converting efficiency of the DC/DC converter should be increased. This is because the converting efficiency of this DC/DC converter is directly reflected onto the power consuming ratio of the cell.
As one method for increasing the converting efficiency of the DC/DC converter, the synchronous rectification type DC/DC converter may be utilized. When this synchronous rectification type DC/DC converter is utilized, the resultant converting efficiency may be increased by approximately 10%, as compared with the conventional type DC/DC converter.
Also, the converting efficiency of the DC/DC converter may be influenced by the performance of a capacitor employed in this DC/DC converter. For instance, in the current DC/DC converters, signals are oscillated in high frequencies in order to increase the converting efficiency and also-to make the entire appliance compact. In such a high-frequency type DC/DC converter, a smoothing capacitor is required in an output unit thereof so as to reduce phase errors.
A smoothing capacitor includes an equivalent series resistor (ESR). When the resistance value of this equivalent series resistor (ESR) is large, the converting efficiency of the DC/DC converter would be deteriorated.
Therefore, such a capacitor is required, in which an equivalent series resistor (ESR) having a small resistance value is included, in order to increase the converting efficiency Qf the DC/DC converter.
As the capacitor in which the equivalent series resistor having the small resistance value is included, there is an organic capacitor.
When an organic capacitor is employed as a smoothing capacitor, since the converting efficiency of the DC/DC converter is increased, heat dissipation is reduced even when a large current flows. As a result, the DC/DC converter with employment of the organic capacitor may be utilized as such a DC/DC converter capable of allowing large currents, for instance, 3 to 5 amp.
In the case that the DC/DC converter with using the organic capacitor as the smoothing capacitor is employed in the apparatus capable of allowing large currents, it is preferable to use an organic capacitor capable of allowing high ripple components as the capacitor employed in the input unit of the DC/DC converter.
On the other hand, as explained above, although organic capacitors own great merits such as high frequency characteristics and temperature characteristics, these organic capacitors have such demerits that-the organic capacitors are easily destroyed by receiving overvoltages, resulting in fire and smoke explosion.
Accordingly, when an organic capacitor is employed in a DC/DC converter, an overvoltage protection mechanism is necessarily required for this organic capacitor.
As the factors why overvoltages are produced in DC/DC converters, various cases may be conceived. That is, the output voltage is increased due to circuit failure of this DC/DC converter. Also, since the cells and the recharging devices malfunction, or improper cells/recharging devices are employed, the voltage inputted into the DC/DC converter is increased to produce the overvoltage.
First, when the output voltage of the DC/DC converter becomes the overvoltage, the smoothing capacitor employed in the output unit of this DC/DC converter must be protected.
As the method for protecting the smoothing capacitor, a zener diode is employed in the output unit of the DC/DC converter. In this method, when the voltage outputted from the DC/DC converter exceeds the normal voltage of this zener diode, the zener diode would be burned out, so that a shortcircuit is established between this DC/DC converter and the load.
In this shortcircuit case, since the current flow is stopped due to this shortcircuit established between the DC/DC converter and the load, it is possible to prevent the overvoltage from being applied to the organic capacitor.
On the other hand, when the zener diode is used to protect the capacitor, if the zener diode fails in the shortcircuit mode, then this shortcircuited zener diode can having the protection function. To the contrary, when this zener diode fails in the opencircuit mode, the organic capacitor provided in the output unit of the DC/DC converter would be burned out, resulting in fire and smoke explosion.
Furthermore, it is practically impossible to define whether the zener diode fails in the opencircuit mode, or the shortcircuit mode. As a consequence, it is improper to utilize such a zener diode as the protection circuit of the DC/DC converter.
On the other hand, in order to prevent the organic capacitor from being burned out by receiving the overvoltage, it is conceivable to employ such a method for using a high withstanding voltage type organic capacitor. However, since such a high withstanding voltage type organic capacitor owns a small capacitance, a plurality of organic capacitors are necessarily employed so as to obtain a desired capacitance. Accordingly, the overall circuit would be made bulky. In addition, there is another problem that since the high withstanding voltage organic capacitor owns the high equivalent series resistor (ESR), the converting efficiency of the DC/DC converter would be lowered.
Also, to prevent the organic capacitor from being burned out by receiving the overvoltage, there is another method for providing a burning preventing fuse with each of all organic capacitors used in the DC/DC converter. However, this method has other demerits that a total number of constructive elements of this DC/DC converter is increased, and at the same time, the manufacturing cost of this DC/DC converter is increased. Moreover, when the fuses are provided with these organic capacitors respectively, there is another problem that the converting efficiency of the DC/DC converter is lowered due to the fuse resistance values.
In addition, since other organic capacitors are also employed in the input unit of the DC/DC converter, this organic capacitor must be protected from the overvoltages.